Exploitation of important deposits of serpentine for the asbestos fiber production in the last decades has generated huge quantities of tailings in North America. This material consists mainly of lizardite Mg3Si2O5(OH)4 with minor antigorite (Mg, Fe)3Si2O5(OH)4, brucite Mg(OH)2, magnetite Fe3O4 and awarite Ni5Fe3. Asbestos tailings are an excellent source of magnesium and silica with appreciable amount of nickel (around 0.2%).
By hydrometallurgical treatments, serpentine tailing can be transform in various products, for examples MgCl2, MgSO4, MgO and amorphous silica, which confirm the potential of this resource. As its name suggest, hydrometallurgical processes are closely linked to the use of water. Acidified aqua media allows to solubilize elements but their subsequent evaporation must be limited. Water elimination can require a large amount of energy and consequently makes industrial operation less profitable.
The use of hydrochloric acid as a leach medium to extract magnesium in serpentine is known and has been demonstrated to be effective. In U.S. Pat. No. 2,398,493, serpentine is treated with HCl 20% at ambient pressure to produce magnesium chloride. The brine is purified by addition of calcined serpentine and magnesium oxide to raise the pH until 7.6. The slurry is filtered and the rich magnesium brine is evaporated to obtain a magnesium chloride salt. By conventional electrolysis of magnesium chloride, hydrochloric acid is regenerated by direct synthesis between chlorine gas and an external source of hydrogen.
Although known processes allow to extract magnesium and nickel with dilute hydrochloric acid, only magnesium is recovered. Nickel is considered as an impurities and is eliminated together with iron and magnesium silicate. Also, heating the large amount of ore at elevated temperatures combined with evaporation of water to recuperate magnesium salt involve a high energy cost.
Due to the fluctuations in metals prices and the pressure of market competition, the mining industry is looking to diversify its product offer and to reduce its production cost. A flexible production combined with an economic process are the key to remain financially viable.
Accordingly, there is thus still a need to be provided with an improved process for extracting magnesium from magnesium-bearing ores such as asbestos tailings.